Today's Fibre optic transmission systems are employing recent advances in optical switching technology to provide reconfiguration at the optical layer. The networks created in the photonic domain have evolved from simple point-to-point and ring architectures to a more arbitrary photonic mesh.
The increased use of optical switching promises to alleviate some of the cost of the network by eliminating the need for multiple transponder interfaces. It is also desirable to keep the signals in the optical domain for as much of their transit through the network as possible because of the inherent power efficiency of optical components. Optical components have power dissipation several orders of magnitude smaller than the equivalent functions in the electronic domain. However; it is a practical reality that optical switching, especially those which are cost effective and low power, have switching speeds several orders of magnitude slower than their electrical counterparts. Therefore, although there is a potential savings in cost (both capital and energy), there is a penalty in the performance of such an entirely optical network in terms of reconfiguration speed.
A second problem which is an issue for optical switching is the creation of a so-called make-before-break switch. Electrical switches of this type are common as are their electronic, digital equivalents. Optical switches; however, since they work by splitting and redirecting optical power, make it much more difficult to create such a function.
Prior to the introduction of photonic switches, all reconfiguration had to be performed in the electronic domain. FIG. 1 shows an example of a network element (NE) 2 where all of the switching and routing functions are performed in the electronic domain. In this arrangement, an Any-to-Any electronic switch 4 is used to interconnect N local access client ports and M electro-optic (EO) interfaces. 6. Each EO interface 6 is connected to a transmission fiber pair 8 via a MUX/DeMUX block 10. Each fiber pair 8 provides a bi-directional optical connection to an adjacent NE for a Dense Wavelength Division Multiplexed (DWDM) signal. Thus, for each fibre pair 8 there must be a corresponding EO interface 6 for each wavelength channel of the DWDM signal, such that all directions may be used.
Typically, each EO interface 6 includes a transmitter (Tx) and a receiver (Rx) which are respectively configured to transmit and receive a predetermined wavelength channel. In some cases, the Tx and Rx may be tuneable, but this is not essential, particularly in cases in which passive filter-based MUX/DeMUX blocks 10 are used.
A mesh network can be created by interconnecting many of these NEs with fibre paths 8. Note that optical amplifiers typically used to overcome the loss of the multiplexing and demultiplexing components as well as the loss of the transmission fibres have been omitted to simplify the drawing for the sake of clarity.
The electronic switch 4 provides connectivity between the client access ports where traffic enters and leaves the network and the EO interfaces 6 which send/receive signals through respective fibre pairs 8. As such the switch 4 also provides an interconnection path between EO interfaces 6 connected to respective different fibre pairs 8, and so also provides connectivity for signals to transit the NE 2.
An advantage of this arrangement is high speed of reconfiguration and the ability to set-up new paths and validate their performance before switching to them; a process often called “bridge and switch” in the field. In addition, the transmission between NE's is performed in the optical domain which is commensurate with the goal of using optical transmission technology for its inherent efficiency.
A major disadvantage of this implementation comes in the amount of equipment required to support reconfiguration. In order to have arbitrary re-configurability in this NE, each direction (fiber pair 8) must be equipped with enough EO interfaces (transponders) 6 to support the maximum cross-section of traffic carrying capacity. In addition, this implies that the electronic switch 4 must also support this capacity in a non-blocking fully reconfigurable fashion.
Another disadvantage is the power dissipation of the solution. As pointed out earlier, optical switching, although slower than electronic, is much less power hungry. Also, since all of the switching is performed in the electronic domain, there is the additional inefficiency and added latency of the Optical-to-Electrical and Electrical-to-Optical conversion of all signals transiting the node.
With the introduction of photonic switches, reconfiguration can now be performed in the optical domain. FIGS. 2-4 show examples of network elements where all of the switching and routing functions are performed in the optical domain. A mesh network is created by interconnecting many of these NE's with fibre paths.
There are two categories of optical switch shown in these examples. The first category, as shown in FIG. 2 and FIG. 3, is based on the use of wavelength selective optical switches 12, either alone (as shown in FIG. 2) or together with optical splitters in a wavelength agnostic or space-switch 14, as shown in FIG. 4. The advantages and disadvantages of each will become clear as they are explained.
A main advantage of wavelength selective switching is the minimal interconnect. Both the local client ports and the line ports of a wavelength selective switch (WSS) 12 (FIG. 2) convey DWDM signals, which dramatically reduces the number of fibres required for interconnect. However, since a MUX/DeMUX block 10 must be placed between the WSS 12 and the EO interfaces 6, each interconnect fibre must contain only one copy of each of the DWDM wavelengths which is commonly referred to as wavelength contention. Therefore, re-use of channels must take place across multiple MUX and DeMUX elements.
Another advantage has to do with the means of achieving wavelength selectivity. The WSS 12 can switch individual wavelengths between DWDM ports without intervening fibre (or waveguide). This allows a much wider filter bandwidth in the switch than is possible using a MUX, DeMUX and a switch in between, which leads us to the architecture in FIG. 4.
Using a space switch 16 as shown in FIG. 4 eliminates the problem of wavelength contention but at the expense of many more interconnect fibres. Also, the scale of the space switch 18 can be problematic. Typical systems have channel counts per transmission fibre on the order of 80, such that at a node with 4 directions (each having respective transmit and receive fibers) and adding and dropping 50% of the traffic locally, the switch size must be at least 960×960.
In addition, all of the channels which are transiting the node must pass through the MUX/DeMUX blocks 10 on their way through the space switch 16, introducing filtering losses which are avoided in the WSS architecture of FIGS. 2 and 3.
Hybrid architectures, where one introduces space switching in combination with MUX and DeMUX along with the WSSs are possible which alleviate the wavelength re-use issues while not introducing the full complement of interconnect.
A major advantage of all-optical implementations is in the reduction of the amount of EO equipment. Each NE need only be equipped with enough EO transponders to support the minimum terminating traffic capacity for that node.
A by-product of this is an advantage in the power dissipation of the solution. As pointed out earlier, optical switching, although slower than electronic, is much less power hungry. Since all of the switching is performed in the optical domain, there is no additional inefficiency of the Optical-to-Electrical and Electrical-to-Optical conversion for signals transiting the node.
Disadvantages of an all-optical implementation include the speed of reconfiguration and the inability to set-up new paths and validate their performance before switching to them; often called “blind switching” in the field.
Techniques which enable the elimination of as many EO transponder interfaces as possible while maintaining overall system flexibility and keeping a low switching time for reconfiguration events remain highly desirable.